1. Field of the Invention
This invention relates to compositions for foundry molds and cores; more particularly, to compositions which include glassy metal alloy reinforcement.
2. Background of the Invention
Metal casting, or founding, is a process for producing metal objects by pouring molten metal into a mold which has a cavity of the desired shape and allowing the metal to solidify. If openings or cavities are to be formed in the casting, a core is inserted into the mold before the metal is introduced. In this specification and the appended claims, the term "molds" is intended to include cores as well.
Molds require strength during casting to support molten metal. However, at some time after a solidified skin has formed on the casting, it is desirable that the mold break easily to permit the casting to be removed without forming hot tears or other defects in the casting. Removal of the casting from the mold is called "shakeout."
Although a variety of materials is used in making foundry molds, by far the most common is sand, since it is relatively inexpensive and is sufficiently refractory for most metal castings. Molding sands may be divided into "soft sands," which remain plastic throughout the mold-making process, and "hard sands," which are more rigid and stronger than soft sands. Compressive and tensile strengths of hard sands may be as high as 25 kg/cm.sup.2 and 15 kg/cm.sup.2, respectively.
Various binders, both organic and inorganic, provide to hard sands their strength and rigidity. Organic binders include oils, such as linseed, crude esters and polymers, refined esters and polymers, and soybean; cereals, such as finely ground corn, wheat or rye flour, frequently gelatinized; resins, such as ureaformaldehyde, resole and novolac; protein and pitch. The most common inorganic binders are cement and CO.sub.2 -hardened sodium silicate.
Although soft sands generally can be recycled, hard sands typically are discarded after one use, since residues of cured binder create clumps and fines which interfere with fluidity and gas permeability of the sand composition. Recycling thus requires separating the sand from the other materials and completely replacing the binder. Additional disadvantages of hard sands are high binder costs and, in some cases, reaction between the mold and metal. It is desirable to have molds which provide higher strength with less binder and with recycle capability.
One technique which has been used to strengthen a variety of materials is that of fiber reinforcement, a subject on which much information has been published (see for example Fiber-Reinforced Materials Technology, N. J. Parratt, Van Nostrand Reinhold, London, 1972). A matrix may be strengthened by introducing strong fibers in such a way that the matrix transfers stress from fiber to fiber evenly distributing applied loads and reducing the crack propagation tendency of the homogeneous brittle solid. In general, greater strength is imparted to the matrix by longer reinforcing fibers, particularly when the fibers are in parallel alignment.